1. Field of the Invention
The present invention relates to a process for producing an optical fiber coupler that provides a constant branching ratio over a broad wavelength range.
2. Brief Description of the Prior Art
An optical fiber coupler is an optical component that permits light to be branched or coupled between a plurality of optical fibers. Optical fiber couplers are known to be produced by either a melt drawing process or a polishing process. The melt drawing process is said to be best suited for the production of couplers single-mode fibers (see "Recent Techniques on Optical Fiber Couplers" in Optronics, No. 5, p. 125, 1988). This process starts with stripping part of the coating from a plurality of optical fibers, heating them to fuse the stripped fibers together, forming a bundle by twisting the fibers or arranging them parallel to one another, drawing the bundle under heating until its characteristics such as branching ratio attain predetermined values, and forming a coupling portion.
One of the important applications of optical fiber couplers is in branching of optical communication lines. In optical communications, light emitted at a plurality of wavelengths (e.g., 1.31 .mu.m and 1.55 .mu.m) is transmitted along each line. Thus, it is also desirable for optical fiber couplers to have constant branching characteristics over a broad wavelength range including emission wavelengths such as 1.31 .mu.m and 1.55 .mu.m. However, optical fiber couplers produced by the melt drawing process have the disadvantage that their branching ratio is largely wavelength dependent.
It has been proposed that this problem be resolved by melt drawing optical fibers having different outside diameters on cladding (see Takeuchi et al., "Broad Wavelength Range Optical Fiber Couplers 1" in C-207 of the Preprint for 1989 Autumn National Conference of the Institute of Electronics, Information and Communication Engineers) or by melt drawing optical fibers having different mode field diameters (see "Broad Wavelength Range Optical Fiber Couplers 2" in C-208 of the same preprint).
However, in order to produce optical fiber couplers of a predetermined branching ratio by these prior art methods, it has been necessary to use a plurality of optical fibers having different structures. Hence, it has been difficult to produce a desired optical fiber coupler using common commercially available single-mode optical fibers and this had made it necessary to use optical fibers of special structures.
An alternative process is determined in U.S. Pat. No. 4,798,438.
This process uses two optical fibers of identical structure, one of which is drawn preliminarily to a smaller diameter. The process is shown schematically in FIG. 6, in which numeral 21 refers to an optical fiber, 22a and 22b to a clamper, and 23 to a burner. First, the coating of the optical fiber 21 is partly stripped off. Thereafter, with both ends of the bare portion of the fiber being clamped by the clampers 22a and 22b, tension is applied to the fiber so that it is pulled in opposite directions indicated by arrows 61 and 62 in FIG. 6, whereby the bare portion is drawn as it is heated with the burner 23.
In this process, the diameter of the preliminarily drawn bare portion determines the branching ratio of the optical fiber coupler to be produced and, hence, it is important to draw the bare portion of the fiber with good control and high repeatability. However, the preliminary drawing step involves so many variables in connection with the draw speed, draw length, etc. that it is difficult to attain identically drawn diameters with separate fibers.